1. Field of the Invention
This invention relates to a permanent magnet motor wherein a rotor comprises a rotor core having a plurality of permanent magnets disposed in the interior thereof.
2. Description of the Prior Art
High-power and high-efficiency permanent magnet motors have recently been developed. FIG. 13 shows an arrangement of a stator and a rotor in one of such recently developed permanent magnet motors. A stator 1 comprises a stator core 2 having twelve slots formed therein. Stator windings 1U and 2U of phase U, stator windings 1V and 2V of phase V and stator windings 1W and 2W of phase W are disposed in the slots. A rotor 4 comprises a rotational shaft 5 and a rotor core 6 mounted on the shaft 5, as shown in FIG. 14. The rotor core 6 has four magnet receiving slots 7 formed therein. Four permanent magnets 8 each having an arc-shaped cross section are inserted into the respective magnet receiving slots 7 axially of the rotor core 6 so that a convex side 8a of each permanent magnet 8 is positioned at the outer peripheral side of the rotor core 6. The permanent magnets 8 are so polarized that the north and south poles N and S are arranged alternately. The rotor 4 composed as described above is rotatably mounted on bearings (not shown) and separated from the stator 1 by a predetermined air gap 9.
FIGS. 15 and 16 show magnetic anistropy of the permanent magnet 8. FIG. 15 shows the case where easy magnetization directions B at portions of the permanent magnet 8 are radially concentrated on the center A of the rotor 4. FIG. 16 shows the case where the easy magnetization directions B are substantially parallel to one another and concentrated on a point at infinity. These anistropic characteristics of the permanent magnet are selected according to various applications of the permanent magnet motor.
FIG. 17 shows an inverter controlled power source for the permanent magnet motor. A DC power source 10 is connected to a switching main circuit 11. The switching main circuit 11 comprises three arms 11U, 11V and 11W connected into a three-phase bridge configuration. Each of the arms 11U, 11V, 11W is composed of two transistors 12 and two diodes 13. Nodes of the three pairs of transistors 12 are connected to output lines Ua, Va, Wa of motor phases U, V and W respectively. The output lines Ua, Va, Wa are further connected to stator windings 1U and 2U, 1V and 2V and 1W and 2W, respectively.
The switching main transistors 12 are controlled by a control circuit 14 so that the stator windings 1U, 2U, 1V, 2V, 1W and 2W are energized for energization periods each of which is represented by an electrical angle of 180 degrees or 120 degrees (see FIG. 18), for example. Furthermore, a rotational position detector 15 is provided for detecting a rotational position of the rotor 4, thereby generating a rotational position signal. The rotational position signal is supplied to the control circuit 14, which then obtains a motor drive signal in accordance with the supplied rotational position signal.
In the above-described permanent magnet motor, square or trapezoidal magnetic flux distribution is obtained in the air gap 9 between the stator 1 and the rotor 4. Motor torque T is developed only while a current is flowing in the stator winding as well known in the art and is shown by the following expression (1): EQU T=m.times.K.times.B.times.I (1)
where m is the number of phases of the stator winding, K is a constant with respect to the number of turns of the stator winding, B is the magnetic flux density in the air gap, and I is a winding current. The magnetic flux of the permanent magnet 8 per pole contributes to the motor torque when the stator windings are sequentially energized for the respective periods of 180 degrees, for example. Accordingly, the magnetic flux per pole in the air gap 9 influences the motor performance.
The magnetic flux in the air gap 9 cannot be increased so much in the above-described construction, so that the driving torque of the motor is relatively low. This poses limitation to miniaturization of the motor or an improvement of the drive efficiency. Consequently, permanent magnet machines employing the permanent magnet motors are increased in size.
When the stator windings are sequentially energized for the respective periods of 120 degrees, part of the magnetic flux of the permanent magnet 8 per pole, which corresponds to the electrical angle of 120 degrees, contributes to the motor torque. FIG. 19 shows distribution of the magnetic flux density in the air gap 9 in the prior art. As shown, the magnetic flux caused in the air gap 9 in non-energization periods does not contribute to the driving torque of the motor. The non-energization periods are shown by oblique lines and correspond to the period between the electrical angles of 0 and 30 degrees and the period between the electrical angles of 150 and 180 degrees. Accordingly, the magnetic flux caused by the permanent magnets 8 is not effectively used, which reduces the driving torque of the motor. Consequently, the miniaturization of the motor and the improvement of the drive efficiency are limited.
Furthermore, third, fifth and seventh higher harmonic components are contained in the magnetic flux caused in the air gap 9. Since the configuration of the rotor core 6 reducing the higher harmonic components is limited, the freedom in designing the permanent magnet motor is reduced and the use of the motor is limited. Thus, the higher harmonic components contained in the magnetic flux cannot be easily reduced.
The higher harmonic components contained in the magnetic flux produces higher harmonic energy components in the air gap 9. A so-called cogging torque is developed by the interaction of the higher harmonic energy components and an opening 3a of each slot 3 of the stator core 3. The cogging torque is superimposed on the torque developed by the magnetic flux caused by the fundamental component but does not serve as effective driving torque. Rather, the cogging torque serves to oscillate rotor 4 and the resultant oscillation is transferred to the motor frame or the equipment driven by the motor, producing undesirable oscillation or noise.